The present invention relates to a semiconductor laser, and more particularly to a semiconductor laser having a cavity and emitting a light upon current injection.
Generally, a semiconductor laser is smaller in size than a solid state laser and a gas laser. The semiconductor laser is higher in energy conversion efficiency than the solid state laser and the gas laser. For those reasons, the semiconductor laser has widely been used in various fields. In order to widen the fields to which the semiconductor laser is applied, it is important to realize a further increase in the output of the semiconductor laser. In case of a short wavelength of not more than 1 micrometer, the maximum acceptable output of the semiconductor laser is limited by a maximum value of the acceptable light density range which prevents a facet of a cavity of the semiconductor laser from being broken. In case of a long wavelength of not less than 1 micrometer, the maximum acceptable output of the semiconductor laser is limited by the maximum value of the acceptable heat generation range, wherein the heat generation is mainly due to the current injection. A wavelength band of 1.48 micrometers is an excitation wavelength of an erbium-doped optical fiber amplifier having a high amplification property. Such the optical fiber amplifier needs a high output semiconductor laser. Further, in recent years, data traffic of advanced communication systems or networks such as internet has been on the rapid increase. In this circumstances, it has been required to further increase the channel number of the multiple channels in the wavelength-division multiplex communication. In order to respond to this requirement, it is necessary to increase the output of the semiconductor laser as an excitation source for the erbium-doped optical fiber amplifier so that a saturation output of the erbium-doped optical fiber amplifier is increased.
In case of the semiconductor laser having the wavelength of 1.48 micrometers, it has been known that the length of the cavity is increased to reduce a heat resistance, and that the generated heat is transferred to a heat sink or a stem. A first conventional semiconductor laser is disclosed in IEEE Photonics Technology Letters, vol. 6, No. 1, p. 4, January 1994. The length of the cavity of the semiconductor laser is made long or increased to 1.2 millimeters. A reflectance factor of a front facet of the cavity is 5%. A reflectance factor of a rear facet of the cavity is 95%. A maximum optical output reaches 360 mW. Further, current confinement structures which comprise p-n-p-n thyristors are provided in both sides of an active layer of the semiconductor laser. A width of the active layer is about 2 micrometers, so that the semiconductor laser shows a light emission in a single transverse mode, thereby realizing a highly efficient coupling between the semiconductor laser and an optical fiber.
In order to realize a furthermore increase in the optical output of the semiconductor laser, it is necessary that the length of the cavity of the semiconductor laser is further increased and the heat resistance and a device resistance are reduced for reducing the heat generation. The increase in the length of the cavity of the semiconductor laser causes a reduction in slope efficiency of the semiconductor laser. The reduction in slope efficiency of the semiconductor laser causes an increase in the necessary driving current of the semiconductor laser. FIG. 1 is a diagram illustrative of variations in optical outputs over injection current of three semiconductor lasers having different cavity lengths of 1.2 millimeters, 2.4 millimeters, and 3.6 millimeters, provided that a reflectance factor of a front facet of the cavity is fixed at 5% and a reflectance factor of a rear facet of the cavity is fixed at 95%. From FIG. 1, it is understood that if the reflectance factors of the front and rear facets of the cavity are fixed, then the increase in length of the cavity causes a slight increase in optical output of the semiconductor laser. Namely, the effect of increasing the optical output of the semiconductor laser by the increase in length of the cavity is small as long as the reflectance factors of the front and rear facets of the cavity are fixed.
In order to have solved the above problem, a second conventional semiconductor laser has been proposed, wherein a width of a waveguide of the semiconductor laser is widen to about 100 micrometers for reducing the device resistance, so that the heat generation is reduced, thereby obtaining an optical output in watt-order. This second conventional semiconductor laser is disclosed in Electronics Letters, vol. 35, No. 12, p. 983, June 1999.
The second conventional semiconductor laser is too wide in waveguide width to allow that the semiconductor laser shows a light emission in the single transverse mode. If the semiconductor laser is coupled to an optical fiber, then it is necessary that the semiconductor laser emits light in the single transverse mode. The second conventional semiconductor laser is hard to emit light in the single transverse mode, then it is difficult that the second conventional semiconductor laser is coupled to the optical fiber.
In order to solve this problem, it might be proposed to modify the second conventional semiconductor laser so that the waveguide has a flared structure, wherein the waveguide increases in width toward a front facet, form which a light is emitted, whilst the waveguide decreases in width toward a rear facet which is coupled to the optical fiber. The width of the waveguide at the rear facet is made narrow so as to satisfy the condition for the single transverse mode. The width of the waveguide gradually increases toward the front facet, form which the light is emitted. The flared structure allows the second conventional semiconductor laser to emit the light in the single transverse mode. The modified second conventional semiconductor with the flared structure has a coupling loss of 40% to the single mode optical fiber, provided specific lens systems are essential, which make it difficult to form an optical module.
In the above circumstances, it had been required to develop a novel semiconductor laser free from the above problem.
Accordingly, it is an object of the present invention to provide a novel semiconductor laser free from the above problems.
It is a further object of the present invention to provide a novel semiconductor laser having a long cavity length and a high slope efficiency.
It is a still further object of the present invention to provide a novel semiconductor laser reduced in thermal resistance.
It is yet a further object of the present invention to provide a novel semiconductor laser reduced in device resistance.
It is further more object of the present invention to provide a novel semiconductor laser which reduces a heat generation upon current injection.
It is moreover object of the present invention to provide a novel semiconductor laser improved in heat radiation feature.
It is yet moreover object of the present invention to provide a novel semiconductor laser improved in high output characteristics.
The first present invention provides a semiconductor laser having at least a cavity, wherein the cavity satisfies the following equation:
L=A+Bxc3x97ln(1/Rf)
where A and B are first and second constants, and L is a length of the cavity, and Rf is a reflectance factor of a front facet of the cavity.
The second present invention provides a semiconductor laser having at least a cavity, wherein the cavity satisfies the following equation:
xcex1m=(xc2xdL)xc3x97ln[1/(Rfxc3x97Rr)]
where xcex1m in is the mirror loss of the cavity, L is the length of the cavity, Rf is the reflectance factor of a front facet of the cavity, and the Rr is a reflectance factor of a rear facet of the cavity.
The third present invention provides a semiconductor laser having at least a cavity, wherein a length of the cavity is not less than 1.5 millimeters, and a reflectance factor of a front facet of the cavity is not more than 2%.
The fourth present invention provides a method of designing a cavity of a semiconductor device, wherein the following equation is satisfied:
L=A+Bxc3x97ln(1/Rf)
where A and B are first and second constants, and L is a length of the cavity, and Rf is a reflectance factor of a front facet of the cavity.
The fifth present invention provides a method of designing a cavity of a semiconductor device, wherein the cavity satisfies the following equation:
xcex1m=(xc2xdL)xc3x97ln[1/(Rfxc3x97Rr)]
where xcex1m is the mirror loss of the cavity, L is the length of the cavity, Rf is the reflectance factor of a front facet of the cavity, and the Rr is a reflectance factor of a rear facet of the cavity.
The sixth present invention provides a method of designing a cavity of a semiconductor device, wherein a length of the cavity is not less than 1.5 millimeters, and a reflectance factor of a front facet of the cavity is not more than 2%.
The above and other objects, features and advantages of the present invention will be apparent from the following descriptions.